The invention relates to an apparatus, which can be used for testing the fatigue properties of cords and filaments. The cords or filaments may be tested independently, or as part of a composite embedded in an elastomeric or plastic matrix.
Conventional apparatus for testing the fatigue properties of a filament or cord apply a constant load, while bending or reverse bending a sample of the filament or cord around one or more pulleys or rollers by using a reciprocating driving mechanism. When a reciprocating motion of a filament or cord is applied around pulleys or rollers, the bending load effects a given length of the sample instead of a localized area. When using a standard one or three roll apparatus, the samples subjected to fatigue testing are not subjected to a uniform number of cycles along their is length. The spreading out of the fatigue over this localized area leads to inaccurate fatigue results. The shorter the length of sample, the more accurate will be the results of the test, which will better represent the actual properties of the material.
An important characteristic of the fatigue performance of a material is its fatigue limit. In the prior art, fatigue limit is determined using numerous test at constant stress amplitude. The constant stress amplitude is progressively reduced to localize the fatigue limit. When the limit is approached, the length of time needed to run the standard fatigue test increases accordingly, and such tests are becoming increasingly expensive. Furthermore, close to the fatigue limit, dispersion becomes high and the number of tests for each level of stress amplitude has to be increased to make the determination statistically valid.
Fatigue failure is due to repetitive stresses. The relation between the maximum level of stress S of the repetitive stresses and the number of cycles N is represented by
S=f (logN),
i.e S is a function of the logarithm of N
Where S represents the constant stress amplitude, and N represents the number of cycles to failure.
As S is decreased to the fatigue limit, N increases infinitely. If the cord is maintained below this stress limit, the material will never fail due to fatigue.
It is an object of fatigue testing to determine the fatigue stress limit of a material. In prior art fatigue tests, where steel filaments or cables are tested, for example, the tests are done using a constant level of stress, and a constant cycling time for the repetitive flexing. Also, when steel cables are tested, the steel filaments used to make the cable are in contact with each other, and their movement relative to each other causes the filaments to fret. The fretting may cause the cord, cable of filament to break prematurely, and may give erroneous fatigue data. This fretting is significant, however, only at a large number of cycles, and is negligible at a low number of cycles.
It is an object of this invention to reduce the number of test cycles in fatigue testing.
It is also an object of this invention to provide an apparatus for testing the fatigue properties of a filament or cable which provides more accurate, reproducible results, and minimizes the amount of time needed to obtain the results.
Other objects of the invention will be apparent from the following description and claims.
An apparatus for fatigue testing cord, cable or filamentary material comprises (a) a tensioner (14) for controlling the tension of a cord, cable, or filament (12) to be tested, whereby a first end of the cord, cable or filament (12) is attached to the tensioner (14), (b) a rolling mechanism (30) directly or indirectly attached to said tensioner (14) by cord, cable, or filament (12) for bending the cord, cable, or filament (12), the rolling mechanism (30) comprising a stationary roller (18) and non-stationary roller (20), wherein the non-stationary roller (20) rides against the stationary roller (18), and (c) a crank arm (22) connecting the non-stationary roller (20) to a rotating driving mechanism (24), whereby the crank arm (22) is attached to the driving mechanism (24) at a perimeter thereof, and when the driving mechanism (24) rotates, the crank arm (22) causes the non-stationary roller (20) to move back and forth.
The illustrated apparatus further comprises a bending surface (19) attached to the stationary roller (18) wherein the back and forth movement of the non-stationary (20) roller causes a cord, cable or filament (12) that is to be tested to bend back and forth on the stationary bending surface (19), and a first attachment mechanism (15) for connecting a first end of a cord, cable or filament (12) to be tested to the tensioner (14), and a second attachment mechanism (17) for connecting a second end of a cord, cable or filament (12) to be tested to the non-stationary roller (20).
In an illustrated embodiment, a method for testing the fatigue properties of a cord cable or filament comprises the steps of (a) attaching a first end of the cord, cable or filament (12) to a tensioner (14) to apply a first known and constant or constantly increasing tension to the cord, cable or filament (12) while testing, (b) mounting the cord, cable or filament (12) on a bending surface (19), (c) connecting a second end of the cord, cable or filament (12) to a non-stationary roller (20) that moves in a constant, non-twisting back and forth motion relative to the bending surface (19), (d) causing the non-stationary roller (20) to move back and forth until the cord, cable or filament (12) breaks, (e) determining the fatigue limit by repeating steps (a) through (d) with different constantly increasing tensions by adjusting tensioner (14), and (f) using the data collected and a linear regression equation to calculate the fatigue stress limit of the material.